Absolute continuity of catalytic measure-valued branching processes

Classical super-Brownian motion (SBM) is known to take values in the space of absolutely continuous measures only if d = 1. For d greater than or equa l to 2 its values are almost surely singular with respect to Lebesgue measu re. This result has been generalized to more general motion laws and branch ing laws (yielding different critical dimensions) and also to catalytic SBM . In this paper we study the case of a catalytic measure-valued branching p rocess in R-d With a Feller process xi as motion process, where the branchi ng rate is given by a continuous additive functional of xi, and where also the (critical) branching law may vary in space and time. We provide a simpl e sufficient condition for absolute continuity of the values of this proces s. This criterion is sharp for the classical cases. As a partial converse w e also give a sufficient condition for singularity of the states. (C) 2000 Elsevier Science B.V. All rights reserved.